The present invention relates generally to a method of making plasma polymerized polymer films. More specifically, the present invention relates to making a plasma polymerized polymer film onto a microtextured surface via plasma enhanced chemical deposition with a flash evaporated feed source of a low vapor pressure compound.
As used herein, the term xe2x80x9c(meth)acrylicxe2x80x9d is defined as xe2x80x9cacrylic or methacrylic.xe2x80x9d Also, (meth)acrylate is defined as xe2x80x9cacrylate or methacrylate.xe2x80x9d
As used herein, the term xe2x80x9ccryocondensexe2x80x9d and forms thereof refer to the physical phenomenon of a phase change from a gas phase to a liquid phase upon the gas contacting a surface having a temperature lower than a dew point of the gas.
As used herein, the term xe2x80x9cpolymer precursorxe2x80x9d includes monomers, oligomers, and resins, and combinations thereof. As used herein, the term xe2x80x9cmonomerxe2x80x9d is defined as a molecule of simple structure and low molecular weight that is capable of combining with a number of like or unlike molecules to form a polymer. Examples include, but are not limited to, simple acrylate molecules, for example, hexanedioldiacrylate, or tetraethyleneglycoldiacrylate, styrene, methyl styrene, and combinations thereof. The molecular weight of monomers is generally less than 1000, while for fluorinated monomers, it is generally less than 2000. Substructures such as CH3, t-butyl, and CN can also be included. Monomers may be combined to form oligomers and resins, but do not combine to form other monomers.
As used herein, the term xe2x80x9coligomerxe2x80x9d is defined as a compound molecule of at least two monomers that can be cured by radiation, such as ultraviolet, electron beam, or x-ray, glow discharge ionization, and spontaneous thermally induced curing. Oligomers include low molecular weight resins. Low molecular weight is defined herein as about 1000 to about 20,000 exclusive of fluorinated monomers. Oligomers are usually liquid or easily liquifiable. Oligomers do not combine to form monomers.
As used herein, the term xe2x80x9cresinxe2x80x9d is defined as a compound having a higher molecular weight (generally greater than 20,000) which is generally solid with no definite melting point. Examples include, but are not limited to, polystyrene resin, epoxy polyamine resin, phenolic resin, and acrylic resin (for example, polymethylmethacrylate), and combinations thereof.
The basic process of plasma enhanced chemical vapor deposition (PECVD) is described in THIN FILM PROCESSES, J. L. Vossen, W. Kern, editors, Academic Press, 1978, Part IV, Chapter IV-1 Plasma Deposition of Inorganic Compounds, Chapter IV-2 Glow Discharge Polymerization, herein incorporated by reference. Briefly, a glow discharge plasma is generated on an electrode that may be smooth or have pointed projections. Traditionally, a gas inlet introduces high vapor pressure monomeric gases into the plasma region wherein radicals are formed so that upon subsequent collisions with the substrate, some of the radicals in the monomers chemically bond or cross link (cure) on the substrate. The high vapor pressure monomeric gases include gases of CH4, SiH4, C2H6, C2H2, or gases generated from high vapor pressure liquid, for example styrene (10 torr at 87.4xc2x0 F. (30.8xc2x0 C.)), hexane (100 torr at 60.4xc2x0 F. (15.8xc2x0 C.)), tetramethyldisiloxane (10 torr at 82.9xc2x0 F. (28.3xc2x0 C.), 1,3-dichlorotetramethyldisiloxane (75 torr at 44.6xc2x0 F. (7.0xc2x0 C.)), and combinations thereof that may be evaporated with mild controlled heating. Because these high vapor pressure monomeric gases do not readily cryocondense at ambient or elevated temperatures, deposition rates are low (a few tenths of micrometer/min maximum) relying on radicals chemically bonding to the surface of interest instead of cryocondensation. Remission due to etching of the surface of interest by the plasma competes with reactive deposition. Lower vapor pressure species have not been used in PECVD because heating the higher molecular weight monomers to a temperature sufficient to vaporize them generally causes a reaction prior to vaporization, or metering of the gas becomes difficult to control, either of which is inoperative.
The basic process of flash evaporation is described in U.S. Pat. No. 4,954,371 herein incorporated by reference. This basic process may also be referred to as polymer multi-layer (PML) flash evaporation. Briefly, a radiation polymerizable and/or cross linkable material is supplied at a temperature below a decomposition temperature and polymerization temperature of the material. The material is atomized to droplets having a droplet size ranging from about 1 to about 50 microns. An ultrasonic atomizer is generally used. The droplets are then flash vaporized, under vacuum, by contact with a heated surface above the boiling point of the material, but below the temperature which would cause pyrolysis. The vapor is cryocondensed on a substrate then radiation polymerized or cross linked as a very thin polymer layer.
According to the state of the art of making plasma polymerized films, PECVD and flash evaporation or glow discharge plasma deposition and flash evaporation have not been used in combination. However, plasma treatment of a substrate using a glow discharge plasma generator with inorganic compounds has been used in combination with flash evaporation under a low pressure (vacuum) atmosphere, as reported in J. D. Affinito, M. E. Gross, C. A., Coronado, and P. M. Martin, xe2x80x9cVacuum Deposition Of Polymer Electrolytes On Flexible Substrates,xe2x80x9d Proceedings of the Ninth International Conference on Vacuum Web Coating, November 1995, ed. R. Bakish, Bakish Press 1995, pg. 20-36, and as shown in FIG. 1a. In that system, the plasma generator 100 is used to etch the surface 102 of a moving substrate 104 in preparation to receive the monomeric gaseous output from the flash evaporation 106 that cryocondenses on the etched surface 102 and is then passed by a first curing station (not shown), for example electron beam or ultra-violet radiation, to initiate cross linking and curing. The plasma generator 100 has a housing 108 with a gas inlet 110. The gas may be oxygen, nitrogen, water or an inert gas, for example argon, or combinations thereof. Internally, an electrode 112 that is smooth or having one or more pointed projections 114 produces a glow discharge and makes a plasma with the gas which etches the surface 102. The flash evaporator 106 has a housing 116, with a monomer inlet 118 and an atomizing nozzle 120, for example an ultrasonic atomizer. Flow through the nozzle 120 is atomized into particles or droplets 122 which strike the heated surface 124 whereupon the particles or droplets 122 are flash evaporated into a gas that flows past a series of baffles 126 (optional) to an outlet 128 and cryocondenses on the surface 102. Although other gas flow distribution arrangements have been used, it has been found that the baffles 126 provide adequate gas flow distribution or uniformity while permitting ease of scaling up to large surfaces 102. A curing station (not shown) is located downstream of the flash evaporator 106. The monomer may be an acrylate (FIG. 1b).
These flash evaporation methods have traditionally been used on smooth surfaces or surfaces lacking microtextured features. A disadvantage of traditional PML (polymer multi-layer) flash evaporation methods is that during the time between condensation of the vapor to a liquid film and the radiation cross linking of the liquid film to a solid layer, the liquid tends to flow preferentially to low points and flatter regions because of gravity and surface tension (FIG. 2a) so that the coating surface 150 is geometrically different from the substrate surface 160. Reducing surface temperature can reduce the flow somewhat, but should the monomer freeze, then cross linking is adversely affected. Using higher viscosity monomers is unattractive because of the increased difficulty of degassing, stirring, and dispensing of the monomer.
Many devices have microtextured surfaces, for example, quasi-comer reflector type micro-retroreflectors, diffraction gratings, micro light pipes and/or wave guides, and microchannel flow circuits. The devices are presently made by spin coating or physical vapor deposition (PVD). Physical vapor deposition may be either evaporation or sputtering. With spin coating, surface area coverage is limited and scaling up to large surface areas requires multiple parallel units rather than a larger single unit. Moreover, physical vapor deposition processes are susceptible to pin holes.
Therefore, there is a need for a method for coating devices that have microtextured surfaces with a conformal coating.
The present invention is a method of conformally coating a microtextured surface. The method includes plasma polymerization wherein a polymer precursor is cured during plasma polymerization. The method is a combination of flash evaporation with plasma enhanced chemical vapor deposition (PECVD) that provides the unexpected improvements of conformally coating a microtextured substrate at a rate surprisingly faster than standard PECVD deposition rates.
The conformal coating material may be a polymer precursor, or a mixture of polymer precursor with particle materials. The polymer precursor, particle, or both may be conjugated, or unconjugated.
The method of the present invention includes flash evaporating a polymer precursor forming an evaporate, passing the evaporate to a glow discharge electrode creating a glow discharge polymer precursor plasma from the evaporate, and cryocondensing the glow discharge polymer precursor plasma on a microtextured surface as a condensate, and polymerizing the condensate before the condensate flows, thereby confornally coating the microtextured surface. The crosslinking results from radicals created in the glow discharge plasma.
Accordingly, the present invention provides a method of conformally coating a microtextured surface.